Degree-day models to predict carrot weevil (Coleoptera: Curculionidae) emergence and oviposition in Nova Scotia, Canada

Carrot weevil, Listronotus oregonensis (LeConte) (Coleoptera: Curculionidae), is a pest of carrot (Daucus carota var. sativus Hoffmann; Apiaceae) throughout eastern Canada. Carrot weevil emergence and oviposition were monitored in commercial carrot fields in Nova Scotia. Cumulative degree days were calculated using a base temperature of 7 °C (DD7), and models were developed to predict cumulative emergence and oviposition using nonlinear regression. Cumulative emergence and oviposition were adequately explained as functions of DD7 by a three-parameter sigmoidal Hill equation. Our emergence model predicted initial and peak adult emergence at 35 and 387 DD7, respectively, with oviposition on carrot baits occurring as early as 42 DD7. Models were then validated to evaluate how well they performed. Oviposition on carrot plants began at the fourth true-leaf stage (342 DD7) and continued until eleventh true-leaf stage. Growers using these models can identify their window of opportunity to manage their carrot weevil populations targeting the majority of emerged adults before oviposition begins in the field.

Evaluating the current integrated pest management recommendations in Canada for carrot weevil (Coleoptera: Curculionidae) and carrot rust fly (Diptera: Psilidae)

The primary insect pests in Canadian carrot production are carrot rust fly (Psila rosae (Fabricius); Diptera: Psilidae) and carrot weevil (Listronotus oregonensis (LeConte); Coleoptera: Curculionidae). An integrated pest management programme for these pests exists in Québec and Ontario, where most Canadian carrot (Daucus carota subsp. sativus (Hoffmann) Schübler and Martens; Apiaceae) production occurs. As current carrot insect integrated pest management recommendations are decades old, laboratory and field trials were performed to evaluate the carrot insect integrated pest management recommendations. Carrot weevil populations were evaluated in the laboratory for resistance to the primary product used for control, phosmet. Ontario carrot weevils exhibited negligible mortality when exposed to phosmet compared with > 80% mortality in a susceptible strain. Using data from a carrot integrated pest management programme, weather data was correlated with monitoring and damage data of both pests from historical records. Increased carrot weevil captures were weakly related to increased damage. Carrot weevil damage was reduced by following integrated pest management recommendations in one of three trials. No strong relationship between weather and carrot rust fly captures was identified, suggesting the degree day model for carrot rust fly activity needs revision. In field trials, carrot rust fly damage was negligible despite integrated pest management recommendations for insecticide applications. Future research should include improving carrot weevil monitoring and control and increasing the carrot rust fly action threshold to optimise insecticide applications.

Biology and Management of the Carrot Weevil (Coleoptera: Curculionidae) in North America

The carrot weevil, Listronotus oregonensis (LeConte) (Coleoptera: Curculionidae), is a devastating pest of high value Apiaceous crops like carrots (Daucus carota subsp. sativusHoffm.), parsley (Petroselinum crispum (Mill.) Fuss), and celery (Apium graveolens L.). Although native to North America, it is a serious pest across the Eastern United States and Great Lakes region of the United States and Canada. Females deposit eggs in the petiole of cultivated hosts, and upon hatching, larvae tunnel down through the petiole and into plant roots causing wilting, yellowing, and plant death. Scouting procedures focus mainly on adult activity and require detection of small egg scars on the foliage and crown of the root, or reliance on traps that are only effective at the beginning of the season before the crop emerges. Several avenues of cultural, biological, and chemical control have been explored for this pest, but with limited success. Furthermore, investigation of these management strategies have primarily focused on carrot systems, neglecting other cultivated Apiaceae. Here we present a review of carrot weevil research and highlight key knowledge gaps in the carrot weevil system, which impede our understanding of this insect’s biology and behavioral ecology. Future research addressing these key knowledge gaps will expand our understanding of this pest and contribute to the development and implementation of more effective management strategies.

Prevalence of a nematode castrator of the carrot weevil and impact on fecundity and survival

Bradynema listronoti is a parasitic nematode described from infected specimens of the carrot weevil Listronotus oregonensis. Prevalence of infection by B. listronoti under field conditions was followed over a period of 16 years in an untreated carrot field. Susceptibility of different carrot weevil life stages was evaluated as well as the impact of infection on fecundity and mortality. Gene expression in infected and uninfected carrot weevils was also compared to evaluate the impact of the parasite on the host transcriptome. Prevalence of B. listronoti in carrot weevil populations was sustained over the years ranging from 20 to 63%. All the weevil stages exposed to B. listronoti inoculum were susceptible to infection, larvae being more vulnerable (59 ± 8% infected) compared with pupae (4 ± 3% infected) and adults (7 ± 3% infected). The fecundity of infected female weevils was greatly reduced (60-fold) due to an inhibition of the maturation of the reproductive system. Transcriptomic analyses revealed that this parasitic castration may have been triggered by the inhibition of reproductive hormone production. The B. listronoti–L. oregonensis interaction represents a case of parasitic castration with a unique potential for biological control of an important pest of carrots.